1. Field of the Invention
The present invention relates to optical communication systems using light as a medium. In particular, the invention relates to a multi-channel optical communication system transmitting and receiving a signal for each channel by using a retroreflective device and/or a digital micromirror device, a corner cube being a typical device thereof, to control reflection of light per channel, and relates to an optical transmitting apparatus and an optical receiving apparatus therefor.
2. Description of the Background Art
Because legal restrictions are imposed upon radio communication, efforts are now being concentrated on studies on a communication system using light, on which any legal restrictions have been put, instead of radio waves. One example of such an optical communication system is disclosed in “On Performance Improvement of Light Intensity Modulation Using Attenuated Total Reflection on Corner Cube” by Toshihiro Tsumura et al., Technical Report of IEICE, SANE94-91, SAT94-91, pp. 111–114.
The disclosed communication system is a two-way spatial optical communication system using a laser, having a laser oscillator on one side and a corner cube on the other side. Laser beams emitted from the laser oscillator are modulated by an external signal so that the signal can be recovered from the laser beams received on the receiver side.
The corner cube reflects the incident beams toward the laser oscillator. At this time, it is the peculiar characteristics of the corner cube that it reflects the incoming light from a certain direction back in the same direction. Consequently, the reflected light reaches in the close vicinity of the laser oscillator. A modulator is provided on one plane of the corner cube for modulating the reflected light by controlling the reflection thereupon i.e., reflecting or non-reflecting, or by controlling the direction of the reflection, i.e., in the direction of the light source (retroreflection) or in another direction. The modulator can be driven by an external signal to make the reflected light carry the information. A photo-receiving sensor is provided on the laser oscillator side for receiving the reflected light. A signal transmitted from the corner cube can thus be recovered from an output of the photo-receiving sensor.
Although this communication system accomplishes a two-way optical communication, it has a problem that only one channel can be allocated to each of transmission and reception. Especially, when a image information is to be transmitted by way of the optical communication in a single channel, there arises a problem owing to the amount of the data that a signal interval per one symbol becomes too short, making in the recovering of the signal extremely difficult particularly in an optical communication using a photoelectric transducer element.
Another medium for the multi-channel communication system using light is an optical fiber. The optical fiber enables an efficient optical communication. However, optical fibers must be laid over a wide area, resulting in a problem that an enormous cost is necessary for building an infrastructure. In addition, the optical fiber is not an effective medium for communication with a vehicle and a spatial communication between vehicles.